Correlation of light-scattering measurement and visual ranking of optical surfaces.

Visual ranking of optical surfaces with a Nomarski differential interference contrast microscope correlates well with a modified fractallike dimension and the average intensity of their scattered light ield.

Comparative study of titanium dioxide thin films produced by electron-beam evaporation and by reactive low-voltage ion plating.

Titanium dioxide (TiO(2)) is often used as a high refractive-index material for multilayer optical coatings. However, the optical properties of TiO(2) films depend strongly on the deposition process and its parameters. A comparative study of TiO(2) films fabricated by conventional electron-beam evaporation and by reactive low-voltage ion plating that uses different phases of Ti-O as starting materials is reported. Results on the variability of TiO(2) thin films are analyzed in relation to process parameters. The potential of fabricating high and low refractive-index multilayer stacks with TiO(2) only, by employing two different deposition processes, is presented with a practical example.

Emission spectroscopy of reactive low-voltage ion plating for metal-oxide thin films.

Excited species in the plasma present during reactive low-voltage ion plating (RLVIP) of refractory oxide films are studied by using emission spectroscopy. We believe we have found a higher ratio of atomic to molecular oxygen then reported for earlier analyses that employed a quadrupole mass spectrometer. The application of emission spectroscopy to the RLVIP process and self-actinometry are discussed with examples of stable and unstable processes.

Optical waveguide characterization of dielectric films deposited by reactive low-voltage ion plating.

We determine the quality of single films of various oxides, which are deposited on thermally oxidized silicon wafers by reactive low-voltage ion plating (RLVIP), by measuring their optical waveguide losses. We use a prism coupler for inserting the radiation of a wavelength-selectable He-Ne laser into the waveguide and a CCD camera for imaging the light scattered from the surface of the films. The waveguide losses of the RLVIP films are typically of the order of 1 to 10 dB/cm. Some data obtained for TiO(2) layers on thermally grown SiO(2) and RLVIP SiO(2) seem to confirm the presence of an absorbing boundary layer between RLVIP SiO(2) and TiO(2) that has been found in SiO(2)-TiO(2) multilayers. The waveguide measurements also reveal unusual index gradients in thick (~ 10 µm) single layers of Al(2)O(3) derived from multimode effective index calculations.

Absorption and thermal conductivity of oxide thin films measured by photothermal displacement and reflectance methods.

Photothermal reflectance and photothermal displacement measurements of optical absorption and thermal conductivity are reported for electron-beam-(EB) deposited and ion-plated (IP) thin films of TiO(2), Ta(2)O(5), and ZrO(2). Of the particular set of samples investigated, the EB films have higher absorption than the IP films. The absorption of the EB samples decreases over a period of ~ 90 min on irradiations with an Ar-ion laser of 488-nm wavelength. By contrast, the absorption of the IP samples changes insignificantly or not at all. Photothermal displacement area scans of coating surfaces yield lower defect densities for the IP samples compared with the EB samples for all three oxide materials. The feasibility and limitations of photothermal measurements for thin-film optical and thermal characterizations are discussed.

Comparison of the properties of titanium dioxide films prepared by various techniques.

Fourteen university, government, and industrial laboratories prepared a total of twenty pairs of single-layer titanium dioxide films. Several laboratories analyzed the coatings to determine their optical properties, thickness, surface roughness, absorption, wetting contact angle, and crystalline structure. Wide variations were found in the optical and physical properties of the films, even among films produced by nominally the same deposition techniques.

1.06-microm laser damage of thin film optical coatings: a round-robin experiment involving various pulse lengths and beam diameters.

A number of commercially available optical coatings, both antireflective and high reflective, have been tested for their laser-induced damage threshold, involving a total of eight different laboratories. The results obtained and the experimental methods used at these laboratories were discussed among the participants in this round robin at a minisymposium held at Balzers, Liechtenstein, 9-10 Dec. 1982. A final report of the round robin and a summary of the conclusions drawn at that symposium are presented.

Nonoptical characterization of optical coatings.

Besides the theoretical design of optical interference coatings, the knowledge of their nonoptical properties like chemical composition, structural features, mechanical peculiarities, and environmental stability is often a basic condition for their industrial production. After some remarks on specifications and standards relevant for optical coatings this paper gives a review on various possibilities for nonoptical characterization of optical coatings. Those methods of surface analysis, depth profiling, and electron microscopy available to and widely used by a major coating manufacturer-like Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS), scanning electron microscopy (SEM), energy dispersive (ED) and wavelength dispersive (WD) electron probe microanalysis (EPMA)-are pointed out in more detail and illustrated with examples from daily practice. Other characterization methods, which are less common but very interesting, are also briefly reviewed.

Color variations of AR coatings caused by a leached layer on the substrate.

Color differences of AR-coated prisms were found to be due to a surface layer with a refractive index of n = 1.46 and a geometrical thickness of ~41 nra on the glass substrate (BaK4, n = 1.5688). The existence of this layer is demonstrated by reflection spectroscopy measurements together with numerical calculations of the spectral reflectance of both the uncoated and coated substrates. Auger electron spectroscopy depth profiling analyses reveal that the Ba content of the glass had been leached out completely in the layer. The leaching process was caused by an improper cleaning agent used with the uncoated prisms in the optical shop. In addition to its inherent optical interference effect, the leached substrate surface seems to influence the condensation and growth of the first layer of AR coatings deposited onto it thus causing additional changes of the spectral characteristics of the coating.

Nodular defects in dielectric multilayers and thick single layers.

The investigation of nodular defects in both thick dielectric single layers and multilayer systems by scanning electron microscopy (SEM) reveals the nodules to be of essentially the same shape no matter how many layers were deposited and what materials were used. The SEM micrographs of a thick LaF(3) layer used in IR coatings and of TiO(2)/SiO(2) multilayer systems are presented. Whereas the nodules in the thick fluoride layer are clearly caused by small spherical particles in an underlying layer of Ge, the cause is less obvious with the multilayer nodules. However, micrographs of uncoated glass substrate surfaces obtained by replicating films used for transmission electron microscopy as well as investigations by SEM and electron micro-probe techniques indicated that particular surface defects or contaminations of submicroscopic size act as the nuclei necessary for the formation of nodules. A growth model and possible measures to prevent nodular defects are discussed from a general point of view.

Electron microscopic investigations of cross sections of optical thin films.

In this paper results are given of an electron optical investigation of the cross section of optical single films and multilayers obtained by fracture. Direct observation techniques with scanning microscopy and the surface replica technique with transmission microscopy were used. Scanning microscopy is simple in sample preparation but has relatively low resolution. The highest resolution of microstructural details can be obtained only with the replica technique in the transmission microscope. Photographs of ZnS-, MgF(2)-, ThF(4)-, TiO(2), and SiO(2)-films mainly in the form of multilayers and of a cermet-type sun protection film are shown. Some optical film properties, such as refractive index and light scattering, are strongly influenced by film microstructure.